The invention relates to a heterodyne optical time domain reflectometer (OTDR) for determining the attenuation behaviour of a monomode waveguide (test waveguide) by measurement of the backscattered parts of light pulses transmitted in waveguide. An acousto-optic modulator (AOM) deflects the transmission beam from a laser into the waveguide to be tested at a light frequency which is modulated with the acoustic frequency when the AOM is acoustically energized in a pulse mode. The light pulses backscattered from the test waveguide are superimposed on a local oscillator beam (LO) constituted by the laser beam which traverses the AOM when the AOM is not energized.
The range of fibre-optic measuring systems for localizing faults in waveguides in accordance with the optical time domain reflectometer (OTDR) principle is limited due to the available laser pulse power and the receiver sensitivity. Particularly in the long-wave spectral range around 1300 nm which is used for bridging larger trajectories due to the low waveguide attenuation of approximately 0.5 dB/km, the range of the measuring systems is often too small because only small laser powers are available (several mW in the waveguide) and backscattering intensities are very low. The detection sensitivity is approximately -73 dBm at a bandwidth of 1 MHz (1 .mu.sec pulse duration=100 m resolution in position).
The detection sensitivity can be considerably increased by employing the heterodyning technique. The limiting noise does not originate from the detector but from the signal to be detected and is approximately -97 dBm at the same data as mentioned above. The polarization dependence of the heterodyne reception generally causes a loss of 3 dB so that approximately -94 dBm can be considered as the minimum power to be detected. Since in the case of heterodyne reception the signal to be detected is mixed with a stable local oscillator level (LO beam), a continuously operating laser must be used in this method. Furthermore its line width must be clearly below the detection bandwidth (1 MHz) so as to prevent additional losses. In heterodyne reception, which has further advantages in noise behaviour, a constant frequency shift between the signal and the LO beam must be generated. In heterodyne reception only half the dynamic range in detector electronics is required as compared with a direct receiver because the receiver signal is proportional to the amplitude and not to the power of the optical backscattering signal.
For a further increase of the range of the reflectometer and for smoothing interference (fading) effects many successively measured signal curves can be averaged.
An arrangement of the type described in the opening paragraph is known from DE-OS 35 06 884. The LO beam and the backscattering signals are superimposed by means of a further AOM. However, since the available AOMs have a considerable transmission loss and since in addition considerable coupling losses occur due to the required bulk-optical connections in spite of the complicated and expensive adjustments, it is not possible to realize a heterodyne OTDR with a sufficiently large range by means of the known system. The system-dependent, theoretically possible gain in range of a heterodyne OTDR is eliminated by high attenuations of the single components.